The present work is focused on the synthesis of NiMn ferrite nanoparticles using hydrothermal method. The samples are examined by X-ray diffractometer (XRD), Field emission scanning electron microscope (FESEM), Transmission electron microscope (TEM), Vibrating sample magnetometer (VSM) and LCR controller in order to understand the ferrite structure, crystallite size, surface morphology, grain size, particle size, M-H loop behavior, and the variation of magnetic permeability as a function of frequency and composition. The results indicated that the high saturation magnetization is observed in case of high manganese content. Besides, the permeability versus frequency plots satisfied the Snoke's law.